Plating bath for forming thin resistance layer, method of formation of resistance layer, conductive base with resistance layer, and circuit board material with resistance layer

ABSTRACT

A plating bath, able to form a resistance layer with a uniform thickness distribution on the roughened surface of a conductive base, including nickel ions and sulfamic acid or its salt as essential components and at least one of phosphoric acid, phosphorous acid, hypophosphorous acid, and salts of the same; a conductive base having a thin resistance layer with a stable resistance, and a resistance circuit board material using the same.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a circuit board material with aresistance layer useful for the production of a printed resistancecircuit board, more particularly relates to a plating bath for forming aresistance layer on a conductive base forming that circuit boardmaterial with a resistance layer by plating and a plating method of thesame.

[0003] 2. Description of the Related Art

[0004] A printed circuit board material including resistors (hereinaftercalled a “resistance circuit board material”) is generally provided inthe form of a multilayer body of an insulating substrate and aconductive base with a resistance layer comprised of a resistance layerbonded with that substrate and copper foil or another high conductivitybase bonded with that resistance layer.

[0005] A printed resistance circuit using a resistance circuit boardmaterial is produced by the subtractive method giving insulating regionsfrom which all of the resistance layer and conductive base on theinsulating substrate are removed, resistance regions from which the highconductivity base is removed, and all remaining conductive regions inaccordance with the targeted pattern of the circuit (mask etching).

[0006] In the past, as the material forming the resistance layer, acarbon-based resistance material was generally used. As other metal thinfilms used, nickel electroplating including phosphorus (Japanese PatentPublication (A) No. 48-73762 and Japanese National Publication No.63-500133), nickel electroplating including tin (Japanese PatentPublication (A) No. 54-72468), etc. have been proposed. With these typesof metal thin resistance layers, however, while it is possible to obtaina film with a high sheet resistance by reducing the thickness, ingeneral, if the thickness is reduced, the uniformity of the metal filmis lost and a constant sheet resistance cannot be obtained, so there arelimits to the reduction of thickness.

[0007] That is, in the production of a conductive base with a resistancelayer, a thin resistance layer is formed on a conductive base byelectroplating, but to raise the bonding strength of the conductive basewith a resistance layer with the insulating substrate, the surface ofthe conductive base is roughened, then is plated with the Ni—P etc.serving as resistance layer. With this method, however, since theresistance layer is present on the rough surface of the conductive base,in particular roughened by fine roughened particles, even right afterplating, the uniformity of thickness of the plating is poor and thesheet resistance lacks stability.

[0008] Further, dissolution of part of the resistance layer cannot beavoided since the layer of the conductive base is etched away when usedas a resistance circuit board material. Further, if there is unevennessin thickness in the Ni—P plating resistance layer, there is the defectthat even part of the resistance layer is sacrificed in order tocompletely remove the layer of the conductive base. It was extremelydifficult to stably leave resistors and produce a printed resistancecircuit board. Further, when producing a multilayer printed resistancecircuit board, the printed resistance circuit board is hot pressed.There are therefore the defects that at this time, cracks occur in partsof only the resistance layer (parts where conductive base is etchedaway), the resistance increases, or sometimes the circuit becomes open.

[0009] In forming a resistance layer by such Ni—P alloy plating of theabove patent publications, nickel ions, phosphorous acid ions, andphosphoric acid ions are essential. A plating bath for forming theresistance layer of the former patent publication also includes sulfuricacid ions and chlorine ions. A conductive base with a resistance layerobtained by plating a conductive base by such a bath suffers from unevencolor at the time of plating and variations in the plating layermicroscopically. In a wide material used at the time of mass production(for example, having a width greater than 300 mm), variation easilyoccurs in the plating thickness and phosphorus content in the widthdirection and the fluctuation in resistance of the resistance circuitbecomes greater.

[0010] In the case of a resistance layer made of an Ni—Sn alloy, tinoxides or hydroxides remain on the insulating substrate when etching theresistance layer when forming insulation regions (dissolving the Ni—Sn)and the problem arises of poor insulation. Further, Ni—Cr, Ni—Cr—Al—Si,etc. formed by vapor deposition have been developed for the samepurposes, but problems of cost and productivity and also problems of alow bonding strength with the insulating material have been pointed out.

SUMMARY OF THE INVENTION

[0011] An object of the present invention is to provide a plating bathenabling the formation of a resistance film by a uniform thicknessdistribution even on a rough surface of a roughened conductive base, aconductive base with a resistance layer having a stable resistance thinresistance layer, and a resistance circuit board material using thesame.

[0012] According to a first aspect of the invention, there is provided aplating bath for formation of a thin resistance layer on a surface of aconductive base, wherein the plating bath includes nickel ions andsulfamic acid or its salt as essential components and at least one ofphosphoric acid, phosphorous acid, hypophosphorous acid, and salts ofthe same.

[0013] Preferably, the plating bath further includes at least one ofsulfuric acid, hydrochloric acid., and salts of the same.

[0014] Preferably, the pH is made not more than 6.

[0015] According to a second aspect of the invention, there is provideda method of formation of a thin resistance layer in an above platingbath wherein the thin resistance layer is formed in a range of bathtemperature of 30 to 80° C.

[0016] Furthermore, there is provided a method of formation of a thinresistance layer in an above plating bath wherein the thin resistancelayer is formed in a range of current density of 1 to 30 A/dm².

[0017] Furthermore, there is provided a method of formation of a thinresistance layer in an above plating bath wherein the thin resistancelayer is formed using an insoluble anode.

[0018] According to a third aspect of the invention, there is provided aconductive base with a resistance layer wherein a thin resistance layercomprised of an Ni alloy layer containing 2 to 30 wt % of phosphorus isformed on the surface of the conductive base by a method of formation ofa thin resistance layer in an above plating bath in a range of bathtemperature of 30 to 80° C.

[0019] Furthermore, there is provided a conductive base with aresistance layer wherein a thin resistance layer comprised of an Nialloy layer containing 2 to 30 wt % of phosphorus is formed on thesurface of the conductive base by a method of formation of a thinresistance layer in an above plating bath in a range of current densityof 1 to 30 A/dm².

[0020] Furthermore, there is provided a conductive base with aresistance layer wherein a thin resistance layer comprised of an Nialloy layer containing 2 to 30 wt % of phosphorus is formed on thesurface of the conductive base by a method of formation of a thinresistance layer in an above plating bath using an insoluble anode.

[0021] Preferably, at least the surface on which the resistance layer isformed has a roughness Rz of not more than 3.5 μm.

[0022] According to a fourth aspect of the invention, there is provideda circuit board material with a resistance layer comprised of aninsulating substrate to at least one surface of which an aboveconductive base with a resistance layer is adhered with the resistancelayer at the base at the inside.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] Preferred embodiments of the present invention will be describedin detail below while referring to the attached figures.

[0024] In the present invention, a bath comprised of mostly sulfamicacid is selected so as to obtain microscopically and macroscopicallyuniform electrodeposition of Ni—P. Further, the processability by hotpressing etc. also becomes better compared with a conventional platingbath.

[0025] The plating bath and plating conditions for plating a resistancelayer on a conductive base according to the present invention are asfollows: The concentration of nickel and the concentration of nickelsulfamate should be ranges ordinarily used in sulfamic acid platingbaths. The range of the nickel sulfamate is preferably 300 to 600 g/L.

[0026] The phosphoric acid, phosphorous acid, and hypophosphorous acidto be gradually added to the plating bath may be added as they are ormay be added in the form of sodium salts instead. As the concentrationof phosphorus at this time, a range of 20 to 150 g/L is preferable.Considering however the prevention of crystallization at the time of adrop in solution temperature such as when the facility is not beingoperated, a range of 20 to 100 g/L is preferable.

[0027] The plating bath can be adjusted in pH by using sodium or anothersalt. Further, NaOH or another alkali or sulfamic acid may be added toadjust the pH. The higher the pH, the worse the uniformity of theplating film, so the pH is preferably kept to not more than 6. Further,below 4, there is less fluctuation of the pH—making this morepreferable. The plating bath may also contain boric acid or another pHbuffer to increase the pH stability and stabilize the film compositionand current efficiency more.

[0028] By adding sulfuric acid, hydrochloric acid, or salts of the sameto the plating bath, it is possible to improve the smoothness andprocessability of the plating film. The concentration is suitably 0.1 to30 g/L. If over this, the hardness and internal stress rise.

[0029] A bath temperature of 30 to 80° C. is preferable in that a goodcurrent efficiency and stable phosphorus content are exhibited. If over70° C., however, the sulfamic acid gradually is hydrolyzed, so thetemperature is more preferably not more than 70° C. from the viewpointof the bath lifetime Further, the current efficiency drops the lower thetemperature, so a temperature of at least 45° C. is preferable.

[0030] The current density should be 1 to 30 A/dm². If over this, a dropin the current efficiency or a deterioration of the smoothness mayeasily occur.

[0031] As the anode, it is possible to use a nickel, Ni—P alloy, oranother soluble anode. However, a soluble anode is dissolved andconsumed with long term plating resulting in a change in the distancefrom the cathode (conductive base) and a deterioration of themacroscopic plating thickness distribution. The concentration of nickelin the bath increases due to the difference of current efficiencybetween the anode and cathode. Therefore, the solution has to be drainedand the cost rises. Due to these reasons, an insoluble anode ispreferably used. Note that if using an insoluble anode, the nickel inthe plating bath is reduced, so it is necessary to replenish the nickel.For this, it is preferable to add nickel carbonate or another nickelsalt.

[0032] Further, if using an insoluble anode, hypophosphorous acidchanges to phosphorous acid or phosphoric acid due to an electrolyticreaction, so for stabilization of the deposited film, phosphorous acidor phosphoric acid is superior to hypophosphorous acid.

[0033] In the film of the resistance layer formed, if the phosphoruscontent is 2 to 30 wt %, a high resistance is obtained and theetchability is good as well. In particular, if 8 to 18 wt %, theresistance and etchability are stabilized and there is less fluctuationin resistance due to dissolution after etching of the conductive base(for example, copper film). The thickness should be in the range of 1 nmto 1000 nm. It is possible adjust the resistance to the desired value bythe phosphorus content and layer thickness. Further, as the alloycomponents other than nickel and phosphorus, it is also possible toinclude copper, cobalt, or other elements. Note that after forming theresistance layer, the surface may be treated by zinc, chromate, silane,etc.

[0034] Further, if the surface of the conductive base before plating istoo rough, the surface roughness of the resistance layer formed over itwill become great as well, it will become difficult to uniformly bondthe Ni—P layer, and fluctuations will easily occur in the platingthickness. Further, when used as a resistance circuit board material,during the hot pressing after etching of the substrate material, stresseasily concentrates due to the roughness and cracking easily occurs, sothe surface roughness Rz of the conductive base before plating ispreferably not more than 3.5 μm, particularly preferably not more than2.5 μm from the viewpoint of processability.

[0035] An embodiment of the method of production of a resistance circuitboard material according to the present invention will be explainednext.

[0036] First, a high conductivity base, for example, copper foil, iscovered on one entire side by a masking adhesive sheet, ink, etc. Next,an Ni—P alloy plating layer is formed as a resistance layer on the othersurface. Thereafter, the masking adhesive sheet etc. is peeled off andan insulating substrate is bonded to the resistance layer side by hotpressing, an adhesive, etc. A printed resistance circuit board is formedfrom this resistance circuit board material by forming insulationregions (where all of the resistance layer and conductive base on theinsulating substrate are dissolved away), resistance regions (where thehigh conductivity base is dissolved away), and conductor regions(remainder) by for example the dissolution method. After formation ofthe circuit, the resistance regions and conductor regions are formedwith a protective layer by a liquid or film-like cover coat.

[0037] In the above processing, as the etching solution, it is possibleto use a known etching solution. For example, in the case of copperfoil, ferric chloride, cupric chloride, ammonium persulfate, a chromicacid-sulfuric acid mixture, an ammonia chelate-based etching solution,etc. may be used. The etching solution for an Ni—P resistance layer maybe a copper sulfate-sulfuric acid solution, ferric sulfate-sulfuric acidsolution, ammonium persulfate-sulfuric acid solution, or other knownsolution.

[0038] As the conductive material forming the conductive base with aresistance layer of the present invention, copper foil or copper alloyfilm, aluminum foil, aluminum alloy foil, iron alloy foil, or other foilhaving a high conductivity formed by electrolysis or rolling ispreferable. Copper foil is the best from the viewpoint of etching orrecycling.

[0039] As the insulating substrate, epoxy resin, polyester, polyimide,polyamide imide, or composites of these with glass cloth, phenolresin-paper, epoxy resin-paper, and other laminates, etc. may be used.Further, the above insulating laminates, sheets, or films havingaluminum or iron sheets bonded to them as heat sinks (bonded on oppositesides to surfaces where resistance layers are provided) may also beused. Further, as the insulating substrate, a ceramic sheet, glasssheet, or other inorganic material using an epoxy resin, polyester,polyurethane, polyamide imide, polyimide, and rubber or other resins orrubbers as an adhesive layer may be used.

[0040] In the above explanation, for simplification., reference was madeto a structure comprised of an insulating substrate on one surface ofwhich a resistance layer and conductive base are bonded, but theresistance circuit board material according to the present invention canbe improved or changed structurally. For example, a structure comprisedof an insulating substrate on both surfaces of which a resistance layerand conductive base are bonded or a structure comprised of an insulatingsubstrate on one surface of which a resistance layer and conductivelayer are bonded and on the other surface of which a high conductivitylayer (for formation of conductors and/or electrodes by etching) isbonded are also included.

[0041] Further, the resistance layer may also be formed not only byproviding it on one roughened surface of the conductive base, but alsoon a nonroughened surface or on two roughened or nonroughened surfacesin accordance with the objective of use. The above explanation was madefor the purpose of generally explaining the present invention and has nolimitative meaning at all. The scope of the present invention can bejudged best by reference to the claims.

[0042] The present invention relates to a resistance circuit boardmaterial, a conductive base with a resistance layer to be bonded to aninsulating substrate forming that resistance circuit board material, anda plating bath for forming a resistance layer on the surface of theconductive base forming that conductive base with a resistance layer. Ingeneral, a printed circuit board is comprised of three layers: aninsulating substrate, an electrical resistance layer, and a conductivelayer, but three or more layers are also included in the presentinvention. Further, a resistance circuit board material obtained bystacking a number of these is of course also included.

[0043] Below, the present invention will be explained in further detailwith reference to examples.

EXAMPLES

[0044] The following conductive base was pretreated by immersion in 1:1hydrochloric acid (35%) water at ordinary temperature for 3 minutes,then was plated with a resistance layer. The unevenness of platingappearance was evaluated and the amount of nickel electrodeposition(mg/dm²) as the plating thickness, the content (%) of phosphorus, andthe resistance in a 1 mm□ after formation of the circuit were measured.The results are shown in Table 1. Note that the pH of the plating bathwas adjusted using sulfamic acid and NaOH in the examples and using NaOHin the comparative examples.

Example 1

[0045] A conductive base was masked with a resistance layer made ofroughened electrolytic copper foil having a thickness of 18 μm and amatte surface of a roughness Rz of 2.1 μm. The shiny surface wascompletely masked, while the matte surface was masked leaving 10×10 cm²open. As the anode, a platinum-plated titanium sheet having a surfacearea of 1.5 dm² was used. The matte surface was plated in the followingbath. Nickel sulfamate: 350 g/L H₃BO₃: 35 g/L H₃PO₄: 50 g/L H₃PO₃: 40g/L Bath temperature: 65° C. Current density: 15 A/dm² Time: 30 sec pH:1.0

Example 2

[0046] Plating was performed by the following bath in the same way asExample 1: Nickel sulfamate: 350 g/L NiCl₂.6H₂O: 45 g/L H₃PO₄: 50 g/LH₃PO₂: 40 g/L Bath temperature: 65° C. Current density: 5 A/dm² Time: 30sec pH: 1.3

Example 3

[0047] Plating was performed by the following bath in the same way asExample 1: Nickel sulfamate: 350 g/L H₂SO₄: 5 g/L H₃PO₄: 50 g/L H₃PO₃:40 g/L Bath temperature: 65° C. Current density: 15 A/dm² Time: 30 secpH: 1.1

Example 4

[0048] Plating was performed by the following bath in the same way asExample 1: Nickel sulfamate: 450 g/L H₃PO₂: 50 g/L Temperature: 30° C.Current density: 10 A/dm² Time: 12 sec pH: 4.0

Example 5

[0049] Plating was performed by the following bath in the same way asExample 1: Nickel sulfamate: 350 g/L NaH₂PO₄: 50 g/L H₃PO₃: 120 g/L Bathtemperature: 60° C. Current density: 1 A/dm² Time: 180 sec pH: 1.4

Comparative Example 1

[0050] In the same way as the examples, electrolytic copper foil havinga thickness of 18 μm and a roughened matte surface of a roughness Rz of2.1 μm was used to completely mask the shiny surface and mask the mattesurface leaving 10×10 cm² open. As the anode, a platinum plated titaniumsheet having a surface area of 1.5 dm² was used. The matte surface wasplated in the following bath. NiSO₄.6H₂O: 150 g/L NiCl₂.6H₂O: 45 g/LNiCO₃: 15 g/L H₃PO₄: 50 g/L H₃PO₃: 40 g/L Bath temperature: 75° C.Current density: 5 A/dm² Time: 18 sec pH: 1.1

Comparative Example 2

[0051] Plating was performed by the following bath in the same way asComparative Example 1: NiCO₃.2Ni(OH)₂.4H₂O: 210 g/L H₃PO₂: 100 g/L Bathtemperature: 30° C. Current density: 5 A/dm² Time: 24 sec pH: 3.5

[0052] The results are shown in Table 1. In Table 1, the averagethickness is the average electrodeposition of nickel (mg/dm²). For theplating thickness, the surface was dissolved to obtain the amounts ofdeposition of nickel and phosphorus. Based on these, calibration curveswere prepared by fluorescent X-rays for measurement. Accordingly, thevalues are for the visible surface area. Note that with nickel, 89mg/dm² corresponds to about 1 μm. 3 σ indicates the fluctuation from theaverage value for 10 plated sheets under various conditions at the timeof measurement of N=2 (total N=20) (3 σ average value).

[0053] The copper foil was etched by overlaying the plating resistancelayer side of the resistance circuit board material prepared in theexamples and comparative examples with epoxy-resin impregnated glasscloth, hot pressing this by a lamination press to bond them and therebyprepare a printed circuit board with a resistance layer, then etching byShipley Neutraetch V-1 at 52° C. until the copper color is no longervisible (about 1 to 2 minutes). Further, the resistance layer was etchedby 250 g/L of copper sulfate and 5 mL/L of sulfuric acid at 90° C. Theunit of resistance is ohm/square. TABLE 1 Example Comp. Comp. Ex. 1 Ex.2 Ex. 3 Ex. 4 Ex. 5 Ex. 1 Ex. 2 Uniform good good good good good PoorPoor appearance Average P % 11.6 14.3 11.7 12.6 15.6 17.3 15.5 Average5.5 2.3 6.2 18.0 3.1 1.3 19.2 thickness 3σ P % 3.0 4.0 2.0 3.0 3.0 13.015.0 3σ 3.0 4.0 3.0 1.5 2.0 11.0 7.0 thickness Average 50.0 75.0 53.027.0 80.0 100.0 25.0 resistance 3σ 3.0 4.0 3.0 2.0 5.0 6.0 18.0resistance

[0054] As clear from Table 1, in the examples of the invention, theappearance was uniform. In the comparative examples, however, theappearance included stripes along the direction of flow of the platingsolution. Further, fluctuations occurred in the plating thickness andbecame a cause of fluctuation in resistance. Further, in ComparativeExample 2 with a thick plating, tortoise-shell like hairline fractureswere seen through observation by an ×500 power microscope, but therewere no hairline fractures and the appearance was uniform in Example 4of the same level.

[0055] Regarding the average content of phosphorus, there was no largedifference in the amount of deposition either in the examples of theinvention or the comparative examples, but the fluctuation (3 σ) wassmall in the examples of the invention or about one-quarter that of thecomparative examples. From this, it was learned that the sheetresistance was constant over the entire surface. The fluctuation (3σ) ofthickness of the resistance layer was much smaller compared with thecomparative examples. From this point as well, it was learned that theresistance was constant over the entire surface. Therefore, Examples 1to 5 feature small fluctuations in resistance and superior uniformity.

[0056] Summarizing the effects of the invention, as explained above, itis possible to reduce the fluctuations in appearance, plating thickness,and composition and prepare and provide products with small fluctuationsin resistance.

[0057] While the invention has been described with reference to specificembodiments chosen for purpose of illustration, it should be apparentthat numerous modifications could be made thereto by those skilled inthe art without departing from the basic concept and scope of theinvention.

What is claimed is:
 1. A plating bath for formation of a thin resistancelayer on a surface of a conductive base, wherein said plating bathincludes nickel ions and sulfamic acid or its salt as essentialcomponents and at least one of phosphoric acid, phosphorous acid,hypophosphorous acid, and salts of the same.
 2. A plating bath forformation of a thin resistance layer as set forth in claim 1, whereinsaid plating bath further includes at least one of sulfuric acid,hydrochloric acid, and salts of the same.
 3. A plating bath forformation of a thin resistance layer as set forth in claim 1, wherein apH is made not more than
 6. 4. A plating bath for formation of a thinresistance layer as set forth in claim 2, wherein a pH is made not morethan
 6. 5. A method of formation of a thin resistance layer in a platingbath as set forth in any one of claims 1 to 4, wherein said thinresistance layer is formed in a range of bath temperature of 30 to 80°C.
 6. A method of formation of a thin resistance layer in a plating bathas set forth in any one of claims 1 to 4, wherein said thin resistancelayer is formed in a range of current density of 1 to 30 A/dm².
 7. Amethod of formation of a thin resistance layer in a plating bath as setforth in any one of claims 1 to 4, wherein said thin resistance layer isformed using an insoluble anode.
 8. A conductive base with a resistancelayer wherein a thin resistance layer comprised of an Ni alloy layercontaining 2 to 30 wt % of P is formed on the surface of the conductivebase by a method of formation of a thin resistance layer in a platingbath as set forth in any one of claims 1 to 4 in a range of bathtemperature of 30 to 80° C.
 9. A conductive base with a resistance layerwherein a thin resistance layer comprised of an Ni alloy layercontaining 2 to 30 wt % of P is formed on the surface of the conductivebase by a method of formation of a thin resistance layer in a platingbath as set forth in any one of claims 1 to 4 in a range of currentdensity of 1 to 30 A/dm².
 10. A conductive base with a resistance layerwherein a thin resistance layer comprised of an Ni alloy layercontaining 2 to 30 wt % of P is formed on the surface of the conductivebase by a method of formation of a thin resistance layer in a platingbath as set forth in any one of claims 1 to 4 using an insoluble anode.11. A conductive base with a resistance layer as set forth in claim 8wherein at least the surface on which the resistance layer is formed hasa roughness Rz of not more than 3.5 μm.
 12. A conductive base with aresistance layer as set forth in claim 9 wherein at least the surface onwhich the resistance layer is formed has a roughness Rz of not more than3.5 μm.
 13. A conductive base with a resistance layer as set forth inclaim 10 wherein at least the surface on which the resistance layer isformed has a roughness Rz of not more than 3.5 μm.
 14. A circuit boardmaterial with a resistance layer comprised of an insulating substrate toat least one surface of which a conductive base with a resistance layeras set forth in claim 8 is adhered with the resistance layer at the baseat the inside.
 15. A circuit board material with a resistance layercomprised of an insulating substrate to at least one surface of which aconductive base with a resistance layer as set forth in claim 9 isadhered with the resistance layer at the base at the inside.
 16. Acircuit board material with a resistance layer comprised of aninsulating substrate to at least one surface of which a conductive basewith a resistance layer as set forth in claim 10 is adhered with theresistance layer at the base at the inside.
 17. A circuit board materialwith a resistance layer comprised of an insulating substrate to at leastone surface of which a conductive base with a resistance layer as setforth in claim 11 is adhered with the resistance layer at the base atthe inside.
 18. A circuit board material with a resistance layercomprised of an insulating substrate to at least one surface of which aconductive base with a resistance layer as set forth in claim 12 isadhered with the resistance layer at the base at the inside.
 19. Acircuit board material with a resistance layer comprised of aninsulating substrate to at least one surface of which a conductive basewith a resistance layer as set forth in claim 13 is adhered with theresistance layer at the base at the inside.